The field of this invention is the production of sponge metal from ore oxides, and in particular, the preparation of a hot reducing gas stream utilized to reduce a material such as iron ore.
The concept of passing a reducing gas through iron ore oxides to produce sponge iron for use in making steel is well known in the steel making art. It is known in this art that the reducing gas must be at a high temperature, such as about 1800.degree.-2000.degree. F., in order for the reducing gas to react with the iron ore with commercial efficiency. U.S. Pat. No. 2,900,247 discloses a method of reducing iron ore to produce sponge iron wherein a stream of reducing gas is heated by first separately heating a stream of air and continuously mixing the heated stream of air with a preheated stream of reducing gas to cause a portion of the reducing gas to be burned in order to further heat the reducing gas prior to passing the reducing gas through a bed of iron ore to reduce the ore to sponge iron. The process of U.S. Pat. No. 2,900,247 has been commercially successful. However, the cost of producing reducing gas has escalated and continues to escalate to the point of making the efficiency of use of primary reducing gas a critical factor. Due to the fact that the process of U.S. Pat. No. 2,900,247 requires that a portion of the reducing gas be combusted, the reducing gas efficiency is significantly less than ideal due to the virtual waste of reducing gas constituents as part of the combustion and heating process. A partial solution to the problem of combusting the reducing components of the primary reducing gas stream, and thus expending them, is suggested in U.S. Pat. No. 3,128,174. This patent will be described in more detail in the description of the preferred embodiment. It should be said at this point, however, that U.S. Pat. No. 3,128,174 also spends reducing gas constituents during the step of heating of the reducing gas by utilizing the reducing constituents thereof as part of the combustion process to heat the reducing gas. Both of these patents refer to a batch process for making sponge iron wherein the reducing gas is passed through a closed vessel containing a stationary load of iron ore. However, the same problems regarding the wasting of reduction constituents of the primary reducing gas applies to continuous processes such as that disclosed in U.S. Pat. No. 3,779,741. Other patents considered no more pertinent than the patents just discussed include U.S. Pat. Nos. 2,915,379; 3,136,623; 3,126,624; 3,126,625; 3,189,879; 3,265,113; 3,375,098; 3,423,201; 3,467,368; 3,601,381; 3,617,227; 3,635,456; 3,681,049; 3,684,486; 3,710,808; 3,720,497; 3,748,120; 3,749,386; 3,764,123; 3,765,872; 3,770,421; 3,814,174; 3,816,101; 3,816,102; 3,827,879; 3,836,131; 3,837,813; 3,846,066; 3,890,142; and, 3,904,397.